General GSM Concept

GSM

GSM ConceptGlobal System for Mobile Communication

A GSM network consists of several functional entities, whose functions and interfaces are defined. The GSM network can be divided into following broad parts.The Mobile Station (MS)The Base Station Subsystem (BSS)The Network Switching Subsystem (NSS)The Operation Support Subsystem (OSS)GSM - Architecture

Following is the simple architecture diagram of GSM Network:

GSM - Architecture

The added components of the GSM architecture include the functions of the databases and messaging systems:Home Location Register (HLR)Visitor Location Register (VLR)Equipment Identity Register (EIR)Authentication Center (AuC)SMS Serving Center (SMS SC)Gateway MSC (GMSC)Chargeback Center (CBC)Transcoder and Adaptation Unit (TRAU)

GSM - Architecture

Following is the diagram of GSM Netwrok along with added elements:GSM - Architecture

The MS and the BSS communicate across the Um interface, also known as the air interface or radio link. The BSS communicates with the Network Service Switching center across the A interface.GSM network areas:In a GSM network, the following areas are defined:Cell:Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.Location Area:A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs.MSC/VLR Service Area:The area covered by one MSC is called the MSC/VLR service area.PLMN:The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.

GSM - Architecture

Specifications for different Personal Communication Services (PCS) systems vary among the different PCS networks. The GSM specification is listed below with important characteristics.Modulation:Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also changed the information by demodulating the signal at the receiving end.The GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method.Access Methods:Because radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible.GSM chose a combination of TDMA/FDMA as its method. The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth.

GSM - Specification

One or more carrier frequencies are then assigned to each BS. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time.Transmission Rate:The total symbol rate for GSM at 1 bit per symbol in GMSK produces 270.833 K symbols/second. The gross transmission rate of the time slot is 22.8 Kbps.GSM is a digital system with an over-the-air bit rate of 270 kbps.Frequency Band:Theuplink frequency rangespecified for GSM is 933 - 960 MHz (basic 900 MHz band only). Thedownlink frequency band890 - 915 MHz (basic 900 MHz band only).

GSM - Specification

Channel Spacing:This indicates separation between adjacent carrier frequencies. In GSM, this is 200 kHz.Speech Coding:GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.Duplex Distance:The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.Misc:Frame duration:4.615 mSDuplex Technique:Frequency Division Duplexing (FDD) access mode previously known as WCDMA.Speech channels per RF channel:8.GSM - Specification

GSM distinguishes explicitly between user and equipment and deals with them separately. Besides phone numbers and subscriber and equipment identifiers, several other identifiers have been defined; they are needed for the management of subscriber mobility and for addressing of all the remaining network elements. The most important addresses and identifiers are presented in the following:International Mobile Station Equipment Identity (IMEI):The international mobile station equipment identity (IMEI) uniquely identifies a mobile station internationally. It is a kind of serial number. The IMEI is allocated by the equipment manufacturer and registered by the network operator, who stores it in the EIR. By means of IMEI, one recognizes obsolete, stolen or non-functional equipment.

GSM - Addresses and Identifiers

There are following parts of an IMEI:Type Approval Code (TAC):6 decimal places, centrally assigned.Final Assembly Code (FAC):6 decimal places, assigned by the manufacturer.Serial Number (SNR):6 decimal places, assigned by the manufacturer.Spare (SP):1 decimal place.Thus, IMEI = TAC + FAC + SNR + SP. It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing.GSM - Addresses and Identifiers

International Mobile Subscriber Identity ( IMSI):Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM). A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI.There are following parts of an IMSI:Mobile Country Code (MCC):3 decimal places, internationally standardized.Mobile Network Code (MNC):2 decimal places, for unique identification of mobile network within the country.Mobile Subscriber Identification Number (MSIN):Maximum 10 decimal places, identification number of the subscriber in the home mobile network.


Mobile Subscriber ISDN Number ( MSISDN):Thereal telephone numberof a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM.The MSISDN categories follow the international ISDN number plan and therefore have the following structure:Country Code (CC) :Up to 3 decimal places.National Destination Code (NDC):Typically 2-3 decimal places.Subscriber Number (SN):Maximum 10 decimal places.


Mobile Station Roaming Number ( MSRN):The Mobile Station Roaming Number ( MSRN) is a temporary location dependent ISDN number. It is assigned by the locally responsible VLR to each mobile station in its area. Calls are also routed to the MS by using the MSRN.The MSRN has same structure as the MSISDN.Country Code (CC) :of the visited network.National Destination Code (NDC):of the visited network.Subscriber Number (SN):in the current mobile network.GSM - Addresses and Identifiers

Location Area Identity (LAI):Each LA of an PLMN has its own identifier. The Location Area Identifier (LAI) is also structured hierarchically and internationally unique as follows:Country Code (CC) :3 decimal places.Mobile Network Code (MNC):2 decimal places.Location Area Code (LAC):maximum 5 decimal places or maximum twice 8 bits coded in hexadecimal (LAC < FFFF).Temporary Mobile Subscriber Identity (TMSI):The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR. It is stored on the network side only in the VLR and is not passed to the HLR.Together with the current location area, TMSI allows a subscriber to be identified uniquely and it can consist of up to 4x8 bits.


Local Mobile Subscriber Identity (LMSI):The VLR can assign an additional searching key to each mobile station within its area to accelerate database access. This unique key is called the Local Mobile Subscriber Identity (LMSI). The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR.An LIMSI consists of four octets ( 4 x 8 bits).Cell Identifier (CI):Within an LA, the individual cells are uniquely identified with a cell identifier (CI), maximum 2 x 8 bits. Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way.


The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated from the Mobile Station.Call from Mobile Phone to PSTN:When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place:The MSC/VLR receives the message of a call request.The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated. If the mobile station is not authorized, service will be denied.MSC/VLR analyzes the number and initiates a call setup with the PSTN.MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot).The BSC allocates the traffic channel and passes the information to the mobile station.The called party answers the call and the conversation takes place.The mobile station keeps on taking measurements of the radio channels in the present cell and neighboring cells and passes the information to the BSC. The BSC decides if handover is required, if so, a new traffic channel is allocated to the mobile station and the handover is performed. If handover is not required, the mobile station continues to transmit in the same frequency.

GSM - Operations

Call from PSTN to Mobile Phone:When a PSTN subscriber calls a mobile station, the sequence of events is as follows:The Gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR.The GMSC routes the call to the MSC/VLR.The MSC checks the VLR for the location area of the MS.The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request.The MS responds to the page request.The BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, after the subscriber answers, the speech connection is established.Handover, if required, takes place, as discussed in the earlier case.

GSM - Operations

The MS codes the speech at 13 Kbps for transmission over the radio channel in the given time slot. The BSC converts (or transcodes) the speech to 64 Kbps and sends it over a land link or radio link to the MSC. The MSC then forwards the speech data to the PSTN. In the reverse direction, the speech is received at 64 Kbps rate at the BSC and the BSC does the transcoding to 13 Kbps for radio transmission.In its original form, GSM supports 9.6 Kbps data, which can be transmitted in one TDMA time slot. Over the last few years, many enhancements were done to the GSM standards (GSM Phase 2 and GSM Phase 2+) to provide higher data rates for data applications.

GSM - Operations

The layered model of the GSM architecture integrates and links the peer-to-peer communications between two different systems. The underlying layers satisfy the services of the upper-layer protocols. Notifications are passed from layer to layer to ensure that the information has been properly formatted, transmitted, and received.

GSM - Protocol Stack

The GMS protocol stacks diagram is shown below:


MS Protocols:The signaling protocol in GSM is structured into three general layers, depending on the interface.Layer 1:The physical layer, which uses the channel structures over the air interface.Layer 2:The data-link layer. Across the Um interface, the data-link layer is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used.Layer 3:The third layer of the GSM signaling protocol is divided into three sublayers:Radio Resource management (RR)Mobility Management (MM) andConnection Management (CM).


The MS to BTS Protocols:The RR layer oversees the establishment of a link, both radio and fixed, between the MS and the MSC. The main functional components involved are the MS, the BSS, and the MSC. The RR layer is concerned with the management of an RR-session, which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels, including the allocation of dedicated channels.The MM layer is built on top of the RR layer and handles the functions that arise from the mobility of the subscriber, as well as the authentication and security aspects. Location management is concerned with the procedures that enable the system to know the current location of a powered-on MS so that incoming call routing can be completed.The CM layer is responsible for CC, supplementary service management, and Short Message Service (SMS) management. Each of these may be considered as a separate sublayer within the CM layer. Other functions of the CC sublayer include call establishment, selection of the type of service (including alternating between services during a call), and call release.


BSC Protocols:After the information is passed from the BTS to the BSC, a different set of interfaces is used. The Abis interface is used between the BTS and BSC. At this level, the radio resources at the lower portion of Layer 3 are changed from the RR to the Base Transceiver Station Management (BTSM). The BTS management layer is a relay function at the BTS to the BSC.The RR protocols are responsible for the allocation and reallocation of traffic channels between the MS and the BTS. These services include controlling the initial access to the system, paging for MT calls, the handover of calls between cell sites, power control, and call termination. The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels. The BSC still has some radio resource management in place for the frequency coordination, frequency allocation, and the management of the overall network layer for the Layer 2 interfaces.From the BSC, the relay is using SS7 protocols so the MTP 1-3 is used as the underlying architecture, and the BSS mobile application part or the direct application part is used to communicate from the BSC to the MSC.


MSC Protocols:At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 from the BSC. Here, the equivalent set of radio resources is called the BSS MAP. The BSS MAP/DTAP and the MM and CM are at the upper layers of Layer 3 protocols. This completes the relay process. Through the control-signaling network, the MSCs interact to locate and connect to users throughout the network. Location registers are included in the MSC databases to assist in the role of determining how and whether connections are to be made to roaming users.Each user of a GSM MS is assigned a HLR that is used to contain the user's location and subscribed services. A separate register, the VLR, is used to track the location of a user. As the users roam out of the area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turn uses the control network (which happens to be based on SS7) to signal the HLR of the MS's new location. Through this information, MT calls can be routed to the user by the location information contained in the user's HLR.


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General GSM Concept

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